A/B Testing

Traffic Cop experiments

More complex experiments, such as those that feature full page redesigns, or multi-page user flows, should be implemented using Traffic Cop. Traffic Cop small javascript library which will direct site traffic to different variants in a/b experiments and make sure a visitor always sees the same variation.

It’s possible to test more than 2 variants.

Traffic Cop sends users to experiments and then we use Google Analytics (GA) to analyze which variation is more successful. (If the user has DNT enabled they do not participate in experiments.)

All a/b tests should have a mana page detailing the experiment and recording the results.

Coding the variants

Traffic cop supports two methods of a/b testing. Executing different on page javascript or redirecting to the same URL with a query string appended. We mostly use the redirect method in bedrock. This makes testing easier.

Create a variation view for the a/b test.

The view can handle the URL redirect in one of two ways:

  1. the same page, with some different content based on the variation variable

  2. a totally different page

Content variation

Useful for small focused tests.

This is explained on the variation view page.

New page

Useful for large page changes where content and assets are dramatically different.

Create the variant page like you would a new page. Make sure it is noindex and does not have a canonical URL.

{% block canonical_urls %}<meta name="robots" content="noindex,follow">{% endblock %}

Configure as explained on the variation view page.

Traffic Cop

Create a .js file where you initialize Traffic Cop and include that in the experiments block in the template that will be doing the redirection. Wrap the extra js include in a switch.

{% block experiments %}
  {% if switch('experiment-berlin-video', ['de']) %}
    {{ js_bundle('firefox_new_berlin_experiment') }}
  {% endif %}
{% endblock %}


See the traffic cop section of the switch docs for instructions.

Recording the data


If you are measuring installs as part of your experiment be sure to configure custom stub attribution as well.

Including the data-ex-variant and data-ex-name in the analytics reporting will add the test to an auto generated report in GA. The variable values may be provided by the analytics team.

if (href.indexOf('v=a') !== -1) {
    // UA
        'data-ex-variant': 'de-page',
        'data-ex-name': 'Berlin-Campaign-Landing-Page'
    // GA4
        event: 'experiment_view',
        id: 'Berlin-Campaign-Landing-Page',
        variant: 'de-page',
} else if (href.indexOf('v=b') !== -1) {
    // UA
        'data-ex-variant': 'campaign-page',
        'data-ex-name': 'Berlin-Campaign-Landing-Page'
    // GA4
        event: 'experiment_view',
        id: 'Berlin-Campaign-Landing-Page',
        variant: 'campaign-page',

Make sure any buttons and interaction which are being compared as part of the test will report into GA.

Viewing the data

The data-ex-name and data-ex-variant are encoded in Google Analytics as custom dimensions 69 and 70.

Create a custom report.

Set the “Metrics Group” to include Sessions. Configure additional metrics depending on what the experiment was measuring (downloads, events, etc.)

Set the “Dimension Drilldowns to have cd69 in the top position and cd70 in the drilldown position.

View the custom report and drilldown into the experiment with the matching name.


Write some tests for your a/b test. This could be simple or complex depending on the experiment.

Some things to consider checking:

  • Requests for the default (non variant) page call the correct template.

  • Requests for a variant page call the correct template.

  • Locales excluded from the test call the correct (default) template.

A/B Test PRs that might have useful code to reuse

Avoiding experiment collisions

To ensure that Traffic Cop doesn’t overwrite data from any other externally controlled experiments (for example Ad campaign tests, or in-product Firefox experiments), you can use the experiment-utils helper to decide whether or not Traffic Cop should initiate.

import TrafficCop = from '@mozmeao/trafficcop';
import { isApprovedToRun } from '../../base/experiment-utils.es6';

if (isApprovedToRun()) {
    const cop = new TrafficCop({
        id: 'experiment-name',
        variations: {
            'entrypoint_experiment=experiment-name&entrypoint_variation=a': 10,
            'entrypoint_experiment=experiment-name&entrypoint_variation=b': 10


The isApprovedToRun() function will check the page URL’s query parameters against a list of well-known experimental params, and return false if any of those params are found. It will also check for some other cases where we do not want to run experiments, such as if the page is being opened in an automated testing environment.